The Earth Science Picture of the Day (EPOD) highlights the diverse processes and phenomena which shape our planet and our lives. EPOD will collect and archive photos, imagery, graphics, and artwork with short explanatory captions and links exemplifying features within the Earth system. The community is invited to contribute digital imagery, short captions and relevant links.

Snowy Mountain Ice Cave

The photo above shows an ice cave located near Crystal Lake in the Big Snowy Mountains of central Montana. The columns behind me are solid ice -- I'm also standing on ice. Most, perhaps all,ice caves in Montana are found in Madison Limestone. This formation is peppered with caves and sinkholes. In fact, Montana's most famous cave, the Lewis and Clark Caverns, was formed in Madison Limestone. Contrary to what the name may imply, ice caves aren't formed in ice, but rather an ice cave is a cave that stays cold enough to maintain ice all year long. The air temperature in caves is typically cooler than the air outside during summer but warmer than the outside air during winter. This is because caves are well insulated by the thickness of rock around them; however, very few stay cold enough for ice to persist throughout the summer months.

The density of air plays a crucial role in the persistence of ice in some of Montana's caves. Molecules of cold air are closer together and move slower than those in warmer air. This makes cold air denser (heavier by volume) than warmer air. As a result, cold air tends to sink to the lowest point -- in this case the bottom of the cave. Additionally, like a deep freezer compartment that opens from the top, ice caves are configured in such a way that their entrances are located higher than their deepest caverns. Thus, cold (dense) air that flows downward cannot flow back out. Instead, this denser air sits undisturbed, allowing ice that formed during the winter to remain intact during summer's hottest days.

The illustration shows that in an ice cave (left) there's no outlet for the cold air. In a "normal" cave (right) cold air spills out and is replaced by warmer air throughout the spring and summer. Photo taken August 19, 2009.